Atrial fibrillation is the most common cause of clinically significant sustained tachycardia and occurs in up to 12% of individuals that are over age 75. Incidence increases with each decade of life. Furthermore, atrial fibrillation is responsible for up to 1/3 of thromboembolic strokes. The selection of available antiarrhythmic drugs and their efficacy is limited. Accordingly, there is an important need for the development of new antiarrhythmic agents that are safe in patients with structural heart disease and are absence of significant negative chronotropic effects. One such drug is Azimilide dihydrochloride, a class III antiarrhythmic agent with very weak bradycardic effects at test doses. This study is to test the long-term safety and efficacy of this new antiarrhythmic drug.